1. Field of the Invention
The present invention relates to a nitride semiconductor epitaxial wafer and, in particular, to a nitride semiconductor epitaxial wafer with reduced threading dislocation, and a nitride semiconductor device fabricated by using the nitride semiconductor epitaxial wafer.
2. Related Art
A conventional nitride semiconductor epitaxial wafer is known in which a first buffer layer of AlxGa1-xN, a second buffer layer of AlN, a third buffer layer of GaN, a channel layer of GaN and a supply layer of AlGaN are sequentially formed on a sapphire substrate in this order (e.g., JP-A-2006-351870).
In the nitride semiconductor epitaxial wafer of JP-A-2006-351870, since the first buffer layer of AlxGa1-xN is provided between the sapphire substrate and the second buffer layer, it is possible to reduce warping of the nitride semiconductor epitaxial wafer and to provide a device with low ohmic resistance.
However, the nitride semiconductor epitaxial wafer of JP-A-2006-351870 has a problem as below.
The buffer layers are formed on the hetero-substrate for reducing dislocations generated in the epitaxial growth layer due to difference in lattice constant and in thermal expansion coefficient between the hetero-substrate and the epitaxial growth layer. Nevertheless, the threading dislocations cannot be sufficiently reduced, so that they may cause leak current when the wafer is used to fabricate a high-frequency nitride semiconductor transistor. Therefore, the nitride semiconductor epitaxial wafer of JP-A-2006-351870 is not suitable for the high-frequency nitride semiconductor transistor which requires low leak current. Thus, the threading dislocation thereof is desired to be further reduced.